The present invention relates generally to micromachining methods and corresponding micromechanical structures produced by micromachining. More particularly, the invention relates to micromachined relays for electric current switching and current-interrupting circuit breaker technology, arrays of micromechanical electric switches which may be used to replace a conventional, relatively bulky, current-interrupting electric switch or relay device, micromechanical structures which include a plunger element free to reciprocate within a cavity, and micromachining methods for forming such structures, and micromachining methods which are compatible with fabrication technologies used to make multi-chip electronic modules.
The subject invention is related to the invention of commonly-assigned Bagepalli et al. application Ser. No. 08/000,313, filed concurrently herewith, entitled "Current Interrupting Device Using Micromechanical Components", the entire disclosure of which is hereby expressly incorporated by reference.
Micromachining is a recent technology for fabricating micromechanical moving structures . In general, semiconductor batch fabrication techniques are employed to achieve what is in effect three-dimensional machining of single-crystal and polycrystalline silicon and silicon dielectrics, producing such structures as micromotors and microsensors . Thus, except for selective deposition and removal of materials on a substrate, conventional assembly operations are not involved. By way of example, a microsensor is disclosed in Haritonidis et al U.S. Pat. No. 4,896,098; and an electrostatic micromotor is disclosed in Howe et al U.S. Pat. Nos. 4,943,750 and 4,997,521.
A temporary structure known alternatively as a release layer or as a sacrificial layer is critical for micromachining because it allows moving parts to be formed by self-registered casting methods, with subsequent selective etching to remove the sacrificial layer. Since micromachining originated from the technology of silicon integrated circuit processing, low-temperature SiO.sub.2 is commonly employed as the sacrificial layer material.
Silicon is not the only material that has been employed in micromachining. Micromechanical properties of sputtered molybdenum are discussed in the paper by Richard B. Brown, Muh-Ling Ger and Tri Nguyen, "Characteristics of Molybdenum Thin Films for Micromechanical Structures", Proceedings IEEE Microelectromechanical Systems, An Investigation of Microstructures, Sensors, Actuators, Machines and Robots, Napa Valley, Calif., Feb. 11-14, 1990 (IEEE Cat. No. 90CH2832-4). Brown et al also suggest use of sputtered aluminum as a sacrificial layer in order to increase etch selectivity and to avoid cracks in molybdenum cantilever beams due to compressive forces originating in an SiO.sub.2 release layer.
As noted above, the invention further relates to arrays of micromechanical electric switches. As is well-known, electrical circuit breakers are used to protect industrial and domestic electrical equipment from damage due to excessive current caused by, for example, short circuits. The damage is avoided or at least minimized by use of circuit breakers which interrupt the flow of current quickly and stop the buildup of thermal energy within a system. Most such circuit breakers are bulky electromechanical switches, and are capable of interrupting a current overload by actuation of a heavy trip mechanism when the overload current is sensed by separate control circuitry. Such breakers require a large contact closure force to be maintained in order to prevent "popping", which is a current-induced repulsive contact force proportional to the square of the current. Accordingly, these devices require substantial amounts of energy to operate since the contacts are necessarily heavy for structural stability, and are to be moved apart at a high rate of speed. Manufacturing complexities and material quantities employed are an inconvenience when these devices are required. Similar considerations apply to relays for current switching relays for control purposes in general.
In this regard, the micromechanical electric switches of the invention may be employed in the context of Hybrid Arcless Limiting Technology (HALT) which achieves arcless interruption of high currents (e.g. interruption of a 70,000 ampere peak, 480 volt, 60 hertz short circuit in less than 200 microseconds) by employing fast-opening metallic contacts for steady-state current carrying, and by rapidly transferring fault current to associated circuitry including energy-absorbing devices and solid state interrupting devices while the metallic contacts are open, such that the destructive energy of arcing is eliminated. This HALT technology is disclosed for example in Howell U.S. Pat. No. 4,700,256, entitled "Solid State Current Limiting Circuit Interrupter"; Howell U.S. Pat. No. 4,698,607, entitled "High Speed Contact Driver for Circuit Interruption Device"; Howell et al U.S. Pat. No. 4,658,227, entitled "High Speed Magnetic Contact Driver"; Howell U.S. Pat. Nos. 4,705,923, 4,717,796, 4,717,798, and 4,725,701, each entitled "Low Voltage Vacuum Circuit Interrupter" and Howell U.S. Pat. No. 4,723,187, entitled "Current Commutation Circuit", each of which is assigned to the present assignee and incorporated herein by reference.
One problem addressed by the invention is how to expeditiously machine a multiple contact switch system which has submillimeter features, high voltage isolation, and electrical resistance &lt;100 .mu.ohm. Conventional machining is impractical because machine tools are limited to larger dimensions and are slow operating because they operate sequentially. Silicon micromachining may be considered, but has several major drawbacks. In particular, cavity depth is limited to a few microns using surface micromachining; in order to form deep cavities, silicon wafer bonding or other exotic techniques would be required. In addition, the relatively thin (e.g. approximately one micron) aluminum electrodes typically employed in the context of silicon micromachining have limited current density, and moreover are susceptible to electromigration and corrosion.
Also related to the invention is what is known as high density interconnect (HDI) technology for multi-chip module packaging, such as is disclosed in Eichelberger et al U.S. Pat. No. 4,783,695, which is incorporated herein by reference. Very briefly, in systems employing this high density interconnect structure, various components, such as semiconductor integrated circuit chips, are placed within cavities formed in a ceramic substrate. A multi-layer overcoat structure is then built up to electrically interconnect the components into an actual functioning system. To begin the multi-layer overcoat structure, a polyimide dielectric film, such as Kapton polyimide (available from E. I. Dupont de Nemours & Company, Wilmington, Del.), about 0.5 to 3 mils (12.7 to 76 microns) thick, is laminated across the top of the chips, other components and the substrate, employing Ultem.RTM. polyetherimide resin (available from General Electric Company, Pittsfield, Mass.) or another thermoplastic as an adhesive. The actual as-placed locations of the various components and contact pads thereon are determined by optical sighting, and via holes are adaptively laser drilled in the Kapton film and adhesive layers in alignment with the contact pads on the electronic components. Exemplary laser drilling techniques are disclosed in Eichelberger et al U.S. Pat. Nos. 4,714,516 and 4,894,115; and in Loughran et al U.S. Pat. No. 4,764,485, each of which is incorporated by reference. Such HDI vias are typically on the order of one to two mils (25 to 50 microns) in diameter. A metallization layer is deposited over the Kapton film layer and extends into the via holes to make electrical contact to 2chip contact pads. This metallization layer may be patterned to form individual conductors during the its deposition process, or it may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser which is scanned relative to the substrate to provide an accurately aligned conductor pattern upon completion of the process. Exemplary techniques for patterning the metallization layer are disclosed in Wojnarowski et al U.S. Pat. Nos. 4,780,177 and 4,842,677; and in Eichelberger et al U.S. Pat. No. 4,835,704 which discloses an "Adaptive Lithography System to Provide High Density Interconnect", each of which is incorporated by reference. Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system as disclosed in,U.S. Pat. No. 4,835,704. Additional dielectric and metallization layers are provided as required in order to make all of the desired electrical connections among the chips.
As is described in detail hereinbelow, one aspect of the present invention is the reliable fabrication of moving structures employing a modified HDI technology, including use of a polymer such as a polyimide (e.g. Kapton) as a casting material.
In conjunction with the methods of the subject invention as described hereinbelow, one might consider employing SiO.sub.2 to form a sacrificial layer, since this material has been successfully employed in silicon micromachining. However, such use of SiO.sub.2 has revealed three problems in particular.
One problem is that cracks can develop in the SiO.sub.2 sacrificial layer during thermal processing as a result of the large mismatch of the thermal expansion coefficient (TCE) for the two materials (30 ppm/.degree.C. for polyimide, 0.5 ppm/.degree.C. for SiO.sub.2). These cracks lead to a permanent joint between the moving structure and the fixed base because metal layers on either side fuse to each other through the cracks in the SiO.sub.2 sacrificial layer.
Another problem with using a SiO.sub.2 sacrificial layer is that, to maintain SiO.sub.2 film integrity, thickness must be limited to one or two microns. Thus sacrificial layers must be relatively thin. Although this thickness is sufficient for silicon micromachining, it is less than the spacing of four to eight microns required between sliding surfaces in the HDI-based process of the invention employing a polymer. The need for this spacing is due to the orientation of these surfaces, which are vertical in HDI to take advantage of a polyimide film thickness of approximately fifty microns. These surfaces are rougher than planar surfaces because they are produced by laser drilling or reactive ion etching instead of film deposition. In a cylindrical configuration of the invention, achievement of sliding motion requires that the spacing be greater than the asperity height.
A third problem with a SiO.sub.2 sacrificial layer is the difficulty of removing it after formation of an upper moving structure. Because the etching proceeds laterally, the etching time is impractically long in HDI-based micromachining employing a polymer where the dimensions are approximately ten times larger than in silicon micromachining. During etching, the entire structure is exposed to a corrosive solution, placing stringent requirements on the etching selectivity, i.e., the materials not intended to be etched must withstand very long etch times. In a trial with SiO.sub.2, the structure required immersion in concentrated hydrofluoric acid (HF) for 16 hours to remove the low-temperature SiO.sub.2 (LTO) release layer through a lateral etch distance greater than 200 microns. Such lengthy etch period would also require increased operator attention.